Process of sintering zinc ores



Patented Mar. 6, 1928.

UNITED STATES,

HENRY J. STEHLI, OF CEDAR GROVE, NEW JERSEY.

PROCESS OF SINTERING ZINC OBES.

No Drawing.

Heretofore it has been the usual practice in the production of metalliczinc from zinc sulphide ores to charge fine ores or concentrates into aroasting furnace where the ores are roasted for the elimination ofsulphur before being subjected to a distillation process. While thesulphur can be almost entirely eliminated by prolonged roasting at hightemperatures, the cost of doing this is commercially prohibitive and inpractice there still remains in the roasted ores, after their removalfrom the furnace, somewhere in the neighborhood of 2% or more of sulphur. Such ores are commonly known as dead-roasted ores.

The ores after roasting are mixed with coal, coke or other fuel andsometimes with small amounts of salt and other ingredients. The chargeis'then placed in the retorts of a retort furnace. To each retort thereis attached a condenser. When the furnace containing the retorts isheated in a suitable manner, a reaction takes lace between the carbonand the zinc, whic latter exists in the roasted ore mainly in the formofzinc oxide, forming metallic zinc vapor and carbon monoxide. The zincvapor passes over into the condenser molten metallic zinc, and thecarbon monoxide passes oil throu h avent in the condenser and burns. fnaddition to the zinc present in the roasted ore in the form of zincoxide there is, due to;imperfect roasting, always an appreciable amountof sulphur which exists in the roasted ore in the form of zinc sulphideand zinc sulphate.

The material remaining in the retorts after the distillation iscompleted and known as zinc residue is drawn out and carried out to thedump. Such zinc residue comprises some earthy matter originallycontained in the zinc ore and a very appreciable amount of zinc whichhas not been reduced and driven off because it existed in'the roastedore in the form of sulphide or sulphate of zinc; There is also present arelatively great amount of unburned carbon, and often small values insilver and gold of the original ore contained therein.

A typical analysis of zinc residue would show something llke 10% zincand 30%carbon. Heretofore such zinc residues have been almost invariablywaste, as no cheap where it is condensed into Application filed October9, 1928. Serial No. 140,681.

and efiicient method has been discovered so far for utilizing them.

The foregoing description refers particu: larly to preliminary roastingof zinc ores in a well known way.v Of recent years it has been proposedto treat pre-roasted zinc ores by the sintering process because of itsbeing more economicaland giving a better roasted product. Under suchprocess after the zinc ores have beenprelim'inarily roasted there hasbeen a SllifiClGIll) amount of sulphur left to be used for fuel in thesintering process, say, 8% to 12%, or if the preliminary roasting hasbeen carried beyond such point green zinc sulphides have been added tothe mixture to increase the sulphur content as fuel for sintering. To becertain of having enough sulphur for carrying on the sintermg operationit has been desirable to err on the high side and have invariably alittle too much sulphur in the charge, as when too'small an amount ofsulphur is in a charge it will not ignite or sinter at all. Adisadvantage of this is that because of using sulphur itself as a fuelfor sintering and ecause of having a slight excess of sulphur it has notbeen practical to entirely burn out the sulphur in the sinteringoperation, so that, at best, the sintered product would have aproportion of sulphur remaining in it, say between 1% and 1 Thisremaining sulphur holds up a certain amount of zinc in chemicalcombination and complicates the reactions. in and satisfactory operationof the retorts in the subsequent process for the recoveryof the zincfrom the sinter. Furthermore, the furnaces referred to for pre-roasting,or dead-roasting, zinc ores are of such a character that they convert aconsiderable amount of the sulphur present in the ore from the sulphideinto the sulphate form, and such pre-roasted charge going into thesintering machine invariablycontains such sulphate sulphur,

which greatly complicates and hinders the sintering operation, becausethe decomposition of sulphates is an endothermic reaction and requiresconsiderable heat. It is, therefore, necessary where sulphur is used asthe sintering fuel to not only leave enough sulhide sulphur in thecharge to furnish the eat for sintering the ore, but also to leave acertain additional amount in the charge to furnish the heat required tobreak up the sulphates and expel the sulphur from them, which furtheraggravates the conditions previously referred to and makes it difficultto expel all the sulphur from the sintering charge so as to leave asintered product sufiiciently low in sulphur to give the best results.

The objects of my invention are to desulphurize zinc ores and to producea roasted or sintered zinc product substantially free from zinc sulphideand entirely free from zinc sulphate and of such a porous physicalcharacter as to permit rapid reduction and distillation of its zinccontents in subsequent treatment, as for the recovery of metallic mm.

A further object of my invention is to utilize the residues, from theretorts in which the pro-roasted and sintered zinc ores have beentreated to obtain metallic zinc, as the fuel to be added to thepro-roasted ore in the subsequent sintering process thereof. Suchresidues are commonly known as zinc residues and contain more or lessunburned carbon which I utilize to furnish the heat necessary forcarrying onthe sintering of the preroasted zinc ores whereby sulphur canbe substantially completely eliminated from the sintered product. Afurther advantage of my invention is that the zinc present in the zincresidues used in the sintering step 18 so acted upon during thesintering process as to lose its refractory character and become againamenable to reduction and distillation in the retorts.

In carrying out my invention I pre-roast fine zinc sulphide ore in anyknown way and by preference down to as low a point as is commerciallyfeasible, preferably without the use of fuel in the preroasting furnaceother than the sulphur in the ore itself, in other words autogenousroasting. The zinc ore may be roasted in this way economically to asulphur content which would be too low to furnish sufficient fuel toalone carry on subsequent sintering or blast roasting of such ore, sayto 3% or &% sulphur, but too high to enable the roasted product to beused directly in the distillation furnaces. To such pre-roasted ore Iadd water in the usual way for sintering and also carbonaceous materialand ignite the charge and cause the carbon to burn and the charge tosinter by passing air therethrough. The sulphates present in theroasting material are very readily and efficiently decomposed and brokenup by the carbon, and under such conditions the material or chargesinters very rapidly and makes an unusually excellent sinter from aphysical viewpoint, and most important, because of the comparatively lowsulphur content the material has as fed to the sintering machine, andbecause of the use of carbon to furnish the heat necessary for carryingon the sintering the sulphur can be entirely eliminated from thesintered product. The roasting or sintering of the charge of preroastedore containing carbonaceous material can be carried on in the well knownsintering machines.

The carbonaceous material I use with the preroasted ore to furnish thenecessary heat to reduce the sulphates and sinter the material compriseszinc residues before referred to. After the material comprisin apreroasted zinc ore and zinc residue Iias been sintered, it may be mixedwith the usual coal or coke, salt, etc., and placed in the retorts andheated in a usual or desired way, whereby the oxidized zinc in thesinter will be reduced to metallic zinc and volatilized. The volatilizedzinc will pass into the condenser and be condensed in the form of amolten metallic zinc and the carbon monoxide will be carried off throu hthe opening in the condenser as usual. ecause the sintered zinc ore isfree from sulphur and highly porous, the distillation of the zinc vaporwill be found to be greatly facilitated by my invention, and, also, I amenabled to recover a higher percentage of the zinc contents of the orethan is possible by the former method.

The zinc residues (from the retorts) which contain carbon and haveheretofore been looked upon largely as a waste product at zinc smelters,furnish an ideal cheap material for supplying the carbon necessary forcarryin on the above-mentioned reducing of sulp iates and sinteringof'the ore. Zinc residues as produced at the zinc smelters may varyconsiderably in composition and all zinc residues in the exact conditionthat they are produced may not be suitable for my purpose. It is wellknown that by screening, by water concentration, or by magneticconcentration, zinc residues in certain cases may be so screened orconcentrated as to produce two or more products, one of which willcontain larger proportions of carbon and less of the earthy materialthan the other and may be more suitable for use as fuel in the sinteringcharge by my process than unscreened or unconcentrated residues, andthe" other or others of which may be suitable for use for other purposesbut may not be desirable in my process. As an illustration, some zincresidues contain an excessive proportion of silica and alumina which itmay be undesirable to return into the sintered charges. By one of theabove described methods it will fre uently be possible to eliminate aconsidcra le proportion of this silica or alumina, thus rendering thezinc residues much more suitable for my purpose. The use of the zincresidues provides advantages since it not only furnishes an easilyavailable and cheap source of carbon but puts into the charge (comrising preroasted zinc ores and zinc residues) a certain amount ofundistilled zinc which is contained in such residues, which zinc becomesavailable for extraction by distillation in the retorts. In other words,the zinc contained in the waste zinc residues from retorts can berecovered when such residues are used in a mixture with preroasted zincores and sintered, since in the sintering process the refractoryundistilled zinc in the residues changes in such a way that it will thenvolatilize in a retort in which the sintered product containin suchpreviously refractory zinc is treate A further advantage of using thezinc residues from the retorts in sintering preroasted zinc ores is thatzinc ores frequently contain small amounts of gold, silver, copper andother metals which have a definite value, and which remain in the zincresidues after the zinc has been distilled, as described in thedistilling process above. Such zinc residues as commonly produced, havegold, silver, copper and other metals in quantities usually too small topay the cost of recovering them. In accordance with my invention ofusing such zinc residues and mixing them into a fresh charge ofpreroasted zinc ore for sintering, the gold, silver, copper and othercontents will gradually become built up in the residues as they are usedone or more times to a point where residues will finally containsuflicient precious and other metal to pay for their recovery. At suchtime the enriched residues from the retorts would be taken out of thecycle for suitable treatment to recover their values, and a new lot ofresidues from the dumps on hand would be used to take their place in thecharges for the sintering machines.

lVhile I have particularly described the desulphurized sintered zinc oreas being adapted to be used as charges in retorts for recovering thezinc therefrom, it will be u11- derstood that such material may be usedfor other purposes than for recovering the metallic zinc therefrom inthe manner stated, such as for the production of zinc oxide from thedesulphurized sinter described, or for the recovery of the zinc as themetal or its salts by other methods, such for example as byhydrometallurgical methods of treatment.

Having now described my invention, what I claim is:

1. In the art of desulphurizing zinc sulphide ores the processconsisting in roastin the ore down to a point where the residue sulphurin the ore, as sulphide, will be insuflicient to carry on sintering ofthe ore, mixing the roasted ore with sufiicient zine residues to supplythe carbon required to furnish the heat and reducing agent to decomposethe sulphates in the ore, and causing internal combustion of saidsulphur and carbonaceous material contained in the zinc residues byblast roasting in an oxidizing atmosphere to eliminate the sulphates inthe ore and sinter the mass.

2. In the art of despuphurizing zinc sulphide ores the processconsisting in roasting the ore down to a point where the residualsulphur in the ore, as sulphide, will be insufiicient to carry onsintering of the ore, mixing the roasted ore with sufiicient zincresidue to supply the carbon required to furnish the heat and reducingagent to decompose the sulphates in the ore, and causing internalcombustion of said sulphur and carbonaceous material contained in thezinc residues by blast roasting in an oxidizing atmosphere to eliminatethe sulphates in the ore and sinter the mass, the heat of combustion ofthe carbon causing such a change in the refractory undistilled zinc inthe said zinc residue as to permit volatilization of said zinc when thesintered product is sub sequently treated in a retort.

3. In the art of desulphurizing zinc sulphide ores .the processconsisting in roasting the ore down to a point where the residualsulphur in the ore, as sulphide, will be insufficient to carry onsintering of the ore, mixing the roasted ore with sufficient zincresidues to supply the carbon required to furnish the heat and reducingagent to decompose the sulphates in the ore, and causing internalcombustion of said sulphur and carbonaceous material contained-in thezinc residues to eliminate the sulphates in the ore, the heat ofcombustion of the carbon causing such a change in the refractoryundistilled zinc in the said zinc residues as to permitvolatilization ofsaid zinc when the I sintered product is subsequently treated in aretort, and causing reduction and volatilization of the zinc in the oreand of the zinc contained in the zinc residue.

4. In the art of desulphurizing zinc sulphide ores the processconsisting in roasting the ore down to a .point where the residual.

sulphur in the ore, as sulphide, will be insufficient to ciirry onsintering of the ore, mixing the roasted ore with sutficient zincresidue to supply the carbon .required to reduce the sulphates in theore and sinter it, and sintering the ore by combustion of the carbon inthe zinc residue and simultaneously causing air to pass through the oreto support combustion and carry ofi' sulphur from the ore.

5. In the art of desulphurizing zinc sulphide ores the processconsisting in roasting the ore down obtained by autogenous roasting bycombustion of the sulphur inthe ore, mixing the roasted ore with zincresidues suflicient to provide the carbon required to furnish thenecessary heat and reagent to react on the to as low a point as can besulphates in the ore and sinter it, and sintering the mixture by causingcombustion of the carbon in the zinc residue and simultaneously causingair to pass through the charge to support combustion and carry offsulphur from the ore.

6. In the art of desulphurizing zinc sulphide ores the processeonsisting in roasting the ore down to as low a point as can be obtainedby autogenous roasting by combustion of the sulphur in the ore, mixingthe roasted ore with zinc residues sufiioient to provide the carbonrequired to furnish the necessary heat and reagent to react on thesulphates in the ore and sinter it, sintering the mixture by causingcombustion of the carbon in the Zinc residue and simultaneously causingair to pass through the charge to support combustion and carry oilsulphur from the ore, the heat of combustion of the carbon causing sucha change in the refractory undistilled zinc in the said zinc residue asto permit volatilization' of said zinc when the sintered product, issubsequently treated in a retort, and causing reduction andvolatilization of the zinc in the ore and of the zinc contained in thezinc residues.

7. In the art of desulphurizing zinc sulphide ores the processconsisting in roasting the ore down to a point where the residualsulphur in the ore, as sulphide, will be insufficient to carry onsintering of the ore, mixing the roasted ore with suiiicient zincresidues to supply the carbon required to furnish the heat and reducingagent to decompose the sulphates in the ore, and cansing internalcombustion of said sulphur and carbonaceous material contained in thezinc residues to eliminate the sulphates in the ore, the heat ofcombustion of the carbon causing such a change in the refractoryundistilled zinc in the said zinc residue as to permit volatilization ofsaid zinc when the sintered product is subsequently treated in a retort,and causing reduction and volatilization of the zinc in the ore and ofthe zinc contained in the zinc residue, adding residues from thereduction and volatilization to preroasted ore and sintering the mixtureto buildup precious metal Values in such sinter.

8. In the art of desulphurizing zinc sulphide ores the processconsisting m roasting the ore down to as low a point as can be obtainedby autogenous roasting by combustion of the sulphur in the ore, mixingthe roasted ore with zinc residues suflicient to provide the carbonrequired to furnish the necessary heat and reagent to react on thesulphates in the ore and sinter it, sintering the mixture by causingcombustion of the carbon in the zinc residue and simultaneously causingair to pass through the charge to SHPPOI'ECOH'I- bustion and carry otl'sulphur from the ore. the heat of combustion of the carbon causing sucha change in the refractory undistilled zinc in the said zinc residue asto permit vol atilization of said zinc when the sintered product issubsequently treated in a retort, and causing reduction androlatilization of the zinc in the ore and of the zinc contained in thezinc residues, adding residues from the reduction and volatilization toprcroasted ore and sintering the mixture to build up precious metalvalues in such sinter.

9. In the art of desulphurizing zinc sulphide ores the processconsisting in roasting the ore down to a point where the residualsulphur in the ore, as sulphide, will be insullicient to carry onsintering of the ore,

mixing the roasted ore with sullicicnt carbonaceous material derivedfrom zinc dcsidues by screening and containing some zinc to supply thecarbon required to furnish the heat and reducing agent to decompose thesulphates in the ore, and causing internal combustion of said sulphurand carbonaceous material contained in the zinc residues to eliminatethe sulphates in the ore.

10. In the art of desulphurizing zinc sulphide ores the processconsisting in roastin the ore down to a point where the residua sulphurin the ore, as sulphide, will be insufiicient to carry on sintering ofthe ore, mixing the roasted ore with suflicient carbonaceous materialderived by concentrating zine residues and containing some zinc tosupply the carbon required to furnish the heat and reducing agent todecompose the sulphates in the ore, and causing internal combustion ofsaid sulphur and carbonaceous material contained in the zinc residues toeliminate the sulphates in the ore.

HENRY J. STEHLI.

